In a large pot or stockpot, sautee onions in oil and butter until tender.Add minced garlic and cook another minute.Add the pumpkin, chicken stock, brown sugar, salt and spices.Cover and simmer 20-25 minutes.Blend the soup using an immersion blender or in batches in a regular blender.Return the mixture to the pot and add the heavy cream.Cook for 5-10 minutes.Garnish with toasted pumpkin seeds, croutons and/or drizzle of heavy cream.